Variable time automatic clothes drier and control therefor



Temperotureflf) Oct. 6, 1953 E J. PETERSON 2,654,160

VARIABLE TIME AUTOMATIC CLOTHES DRIER AND CONTROL THEREFOR Filed March26, 1951 2 Sheets-Sheet 1 Time inuTes) imam M QQIM. I

2 Sheets-Sheet 2 E. J. PETERSON VARIABLE TIME AUTOMATIC CL DRIER ANDCONTROL THEREFOR Oct. 6, 1953 Filed March 26, 1951 [m/emor ElmerJ@Zerrozz filzfys.

av uw Patented Oct. 6, 1953 VARIABLE TIME AUTOMATIC CLOTHES DRIER ANDCONTROL THEREFOR Elmer J. Peterson, Minneapolis, Minn., assignor toFranklin Transformer Manufacturing Company, Inc., Minneapolis, Minn., acorporation of Minnesota Application March 26, 1951, Serial No. 217,586

Claims. 1

My invention relates to an improved automatic clothes dryer and controltherefor characterized by the ability to dry clothes automatically untilpreset condition of dryness is reached regardless of the drying timerequired for this purpose.

Automatic clothes dryers are usually operated on a fixed time cyclewherein the dryer is energized for a preset time and then automaticallystops. While this type of control frees the operator from attending thedryer at all times, it is subject to the disadvantage that it dependsentirely on the judgment of the operator with respect to the timesetting. For example, if the operator has great skill in estimating thequantity of clothes going into the machine, the quantity of waterentrained in the clothes and the characteristics of the clothes withrespect to the difficulty of evaporating the water therefrom, it may bepossible to set the timing of the dryer with a considerable degree ofsuccess. However, if the operator is like most housewives and can onlyestimate the time of drying a particular load in a rough way, greatvariations in the degree of dryness of the clothes are to be expected,some loads coming out of the dryer in an unduly wet condition and otherloads being over-dried with the attendant difilculties of ironing andpossibility of scorching.

The foregoing difliculties are particularly acute in the case of dryersintended for domestic use, for the average housewife is prone to ignoredirections and cannot take measurements of the weight and water contentof the clothes and has neither the interest nor the opportunity todevelop the skill required for the esoteric art of estimating dryingtime by mere inspection.

In accordance with the present invention, the foregoing problem inherentin all fixed time dryers is overcome by providing variable timeenergizing elements sensitive to the degree the clothes are dry andoperative to deenergize the dryer when a preset degree of dryness isreached. These elements, operative in response to temperatures atvarious points in the dryer, sense the condition of the clothes withrespect to dryness and can be set to produce the optimum degree ofdryness under all conditions of load, thereby eliminating entirely theneed for close control, accurate evaluations of the character of theload, and the like heretofore required.

The apparatus of the present invention has the further advantage ofbeing particularly suitable for application to dryers of the typedescribed and claimed in Ralph G. Pugh patent application Serial No.200,069, filed December 9, 1950, entitled Clothes Dryer. This clothesdryer 2 is characterized by provision of a heater, a drying chamber, anaspirator unit and means forming a closed path for air circulationthrough these parts in succession and, as described in the aforesaidPugh application, is particularly efficient in drying.

It is therefore a general object of the present invention to provide anautomatic dryer and control therefor capable of drying clothes to apredetermined degree of dryness. Y Another object of the presentinvention is to provide an improved control mechanism for a dryer of thetype having a heater, a drying chamber, an aspirator unit and meansforming a closed path for air circulation through these parts insuccession and capable of drying clothes to a predetermined degree ofdryness.

Further it is an object of the present invention to provide a clothesdryer and control therefor of the variable time type wherein a simpleand rugged temperature responsive switch senses the degree of dryness ofthe clothes.

Another object of the present invention is to provide a clother dryerand control therefor of the variable time type wherein simple switchingarrangement establishes the circuit connections required for drying andmaintains the same until temperature responsive elements indicate thatthe clothes are dry and deenergize the unit.

Still another object of the present invention is to provide an improveddryer of the type using an aspirator and air heating means automaticallyoperable to dry clothes to a predetermined degree of dryness andthereafter automatically operable to inject aspirator water until themechanism cools.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, together with further objects and advantages thereof, will bestbe understood by reference to the following description taken inconnection with the accompanying drawings, in which:

Figure 1 is a view in perspective of a clothes dryer constructed inaccordance with the present invention with the cabinet and parts brokenaway to show the interior thereof;

Figure 2 is a somewhat diagrammatic circuit diagram of the control ofthe present invention;

Figure 3 is a circuit diagram of an alternative form of the presentinvention; and,

Figure 4 is a chart showing how the temperature varies in the dryer inthe absence of the timing mechanism of the present invention.

Referring now to Figure 1 there is shown at in a tumbling drum forclothes to be dried. This drum has a plurality of inwardly extendingpaddle blades illa which, as the drum is rotated, aid in causing theclothes to partake of the peripheral velocity of the drum. The end iflbof the drum has a circular opening covered by the screen I2 to permitair travel into the drum in the axial'direction. The drum is supportedby a suitable shaft extending backwardly behind end Iflb (not shown) bya plurality of ribs lllc forming a spider with a hub to which the shaftis secured. The forward end id of the drum HI is of annular shape andforms at its center an opening for introducing clothes into the drum orremoving clothes from the drum. Heat losses from the drum are reduced bythe heat insulation it which is held in place by the metal bands Ma.

The forward central access opening of the drum receives a fixed accesstube l5 which, at its upper portion has a rectangular air exit window oraperture lBa An air outlet tube I8 is located over this aperture andextends in slanting direction upwardly and outwardly as shown. At itsend, the tube [8 receives the circular aspirator tube which is orientedin parallel relation to the axis of drum l0 and extends backwardly tothe separator unit 22.

The separator unit 22 extends downwardly from the rear end of the tube20 and defines an air passage extending into the header 24 locateddirectly behind the screen l2. This header is of semi-circular shape andextends close to the screen l2 to assure that practically all the airflows directly from the header 2 into the back end of the drum. Asemi-circular panel 26 extends downwardly from the front end of theheader 24 to seal the portions of screen l2 not located directly overthe header 2!.

The above described structure is disclosed and claimed in Ralph G. Pughapplication Serial No. 200,069, filed December 9, 1950, entitled ClothesDryer. Briefly, its operation is as follows:

The drum in is rotated by motor 28 at a speed sufiicient to cause theclothes to ride against the walls of the drum and to fall in awell-defined descending stream straddled by the air flowing axiallythrough the drum. This airis heated by the electric heating element atin the header 24 so as to reach a condition of low relative humidity toform an effective drying medium. The clothes falling through this airare thereby dried.

Air is withdrawn from the drum l0 through the window I611 in the accesstube i6 and passes into the duct l8 as shown by the arrows. Uponreaching the end of this duct, the air is struck by the water jet 32issuing from the aspirator nozzle 34, this jet being derived from thewater pipe 36, connected to a suitable source of water under pressure(not shown), through the solenoid valve 38. The aspirator jet drives theair into the tube 20, thus producing the air pressure gradient thatdrives air through the closed air circulating path to cause it to passthrough said tumbling drum, aspirator jet, and heater in succession.

In addition to causing air motion, the jet stream 32 cools the moistureladen air from tube Hi to a relatively low temperature. In thiscondition, the water content of the air, even when saturated, is quitelow and water condenses out on the droplets of the aspirator jet. As aconsequence, the air, though saturated at the relatively low temperatureof the jet water, has a low actual moisture content.

The saturated, droplet carrying, cool air issues from duct 20- into theseparator unit 22. This unit defines a U-shaped vertical air passagewhich, at its lower end has an abrupt change of direction of air flowfrom down to up, as shown by the arrows. The heavy droplets of watercannot partake of this abrupt change in velocity and so continue theirdownward movement to the sump portion 22a of the separator. Pump 40draws the water from this sump and discharges it to a suitable drain(not shown).

The air, cool and saturated but carrying no entrained moisture travelsfrom the separator 22 into the short duct 42 leading to the header 24.The air thereupon comes into contact with the heater 30 to be heated toa condition of low relative humidity to form an effective drying medium.From this point on the cycle is repeated.

As pointed out in the above-mentioned patent application, the dryingunit is highly eflicient in operation, particularly with respect topower consumption when operating at optimum speed, and is suitable foruse as a domestic clothes dryer.

Control Heretofore, it has been common practice to provide domesticclothes dryers with timing devices operative to energize the dryer for apredetermined time and then to deenergize the dryer. This type ofcontrol is subject to the disadvantage that it does not automaticallyaccommodate the type or condition of the clothes being dried andconsequently requires an accurate estimatitan of the required dryingtime as the unit is se The apparatus of the present invention differsfrom the fixed-time control heretofore used in providing a variable timecontrol sensitive to the condition of the clothes being dried. In theform shown in Figures 1 and 2, this unit includes a pair of thermostats,one (the cut-off thermostat), indicated generally at 44, beingresponsive to the temperature of the saturated air as it leaves the duct20 and the other (the setting thermostat). indicated generally at 46,being responsive to the temperature of the air as it enters the tumblingdrum [0. The thermostat 46 serves to maintain the unit in the oncondition until the air temperatures reach normal drying values. Thethermostat 44 then takes over to maintain dryer operation until itstemperature falls below a preset value indicative of the end of thedrying cycle.

It has ben found that with the drying cycle of the dryer of Figure 1,the temperature of the air entering the header 24 falls when themoisture has been removed from the clothes. This temperature is believedto fall because the water of jet 32 evaporates as required to bring theair in duct 20 to a saturated condition, an effect that requiresevaporation of a greater quantity of water when the air from duct I8 isdrier than when it is wet. Moreover, the specific heat of the dry airentering the aspirator chamber when the clothes are in a relatively drycondition is less than that of the relatively wet air when the clothesare not dry. Since this evaporation requires heat, the temperatures ofthe air at the outlet of duct 20 falls and a corresponding drop issensed by the thermostat 44.

The circuit diagram of Figure 2 shows the circuit arrangement forachieving variable time automatic drying. Alternating voltage from asuitable source (not shown) is applied to the three terminals 48, 50,and 52 of a conventional three wire system wherein volts, for example,appears across terminals 48 and 50 and across terminals 50 and 52, and220 volts appears across terminals 48 and 52.

The heater 30 is connected across terminals 48 and 52 by the solenoidoperated connector 54. When the solenoid 54a is energized, the circuitfor this purpose may be traced from terminal 52, through conductor 52a,and contact elements 54b, to the heater 30. The opposite end of theheater is connected to terminal 48 through conductor 48a and contactelements 54c. A high temperature operated switch 56 is provided tointerrupt the energizing circuit to heater 30 if the air temperatureexceeds a preset value. This switch may, for example, be located in theduct 42 as shown in Figure 1. Since this element is well known in theart and many wellknown devices may be used for this purpose, it is notshown in detail.

The motor 28, and the water valve 38, Figures 1 and 2, are energized bytwo circuits, one traceable from terminal 48, through conductors 48b and48c'to thermostat 48 and from that thermostat through motor 28 and valve38 to terminal 50. The other circuit may be traced from terminal 48,through conductors 48b and 48d and contactors 54d, to motor 28 and valve38 and then to terminal 50. The motor and valve are energized throughthe latter circuit to initiate operation and through the former circuitduring the later part of the clothes drying operation.

The solenoid 54a is energizable through two circuits. One circuit, usedfor starting, can be traced from terminal 48, through thermostat 48(which in the low temperature starting condition has the dotted lineposition of Figure 2), stop switch 58, solenoid 54a, and start switch80, to conductor 50a and hence to terminal 50. The solenoid 54a is alsoenergized through the sealin circuit traceable from terminal 50',through contactors 54e to solenoid 54a, and through stop switch 58 tothermostat 44 or 46, conductor 48c, and conductor 48b to terminal 48.

The thermostat 46 is of the type operative to close at one temperature,say 89 C., to the position shown in the solid lines of Figure 2 and toopen to the dotted line position at a substantially lower temperature,such as 69 C. One way to accomplish this relatively wide dead band whileachieving positive contact-making is by the use of the over-centerbimetal 48a and a compression spring 46b which biases the bimetal in ageneral longitudinal direction. The bimetal and the spring coact to forma toggle which cannot be driven over-center to the opposite positionuntil a substantial force is exerted, a condition requiring asubstantial temperature change and, when once driven over centerpositively moves to contact-making position. Other methods of achievinga wide dead band may, of course, be used. I

The thermostat 44 is a make or break type thermostat of the bulb type.The sensing bulb 44a is positioned in the separator 22 adjacent theoutlet of duct 20 and is connected by tube 44b to the control unit 62,Figure 1. This unit includes switch arm 44d with which bellows 44ecoacts to make contact when the temperature of bulb 44a is above apreset value. The switch arm engages the fixed arm 441 in thecontactmaking position, the position of arm 44f being determined by theadjusting knob 440 to set the degree of dryness to be imparted to theclothes. Other thermostat devices, operative to open. circuit when apredetermined low temperature exists may be used as the thermostat 44.

As is described in further detail hereafter, the thermostat 44 acts as ashut-off switch.

The switch 48, Figure 2, is closed by closure of the door of the dryer.the indicator light 49a. The unit is protected by fuse 5| which may, forexample, be a 10 ampere fuse.

Operation Operation of the mechanism of Figures 1 and 2 is as follows:

When the unit is in a shut down condition and start button is closed,the solenoid 54a is energized through switch 48 by connection acrossterminals 48 and 50. The solenoid then seals in through the contacts 54cwhich bridge the start button 80 which, upon release, opens.

When the solenoid 54a. picks up, motor 28 and valve 38 are energizedfrom terminals 50 and 48 through the thermostat 46 and the contactors54d. The heater 30 is energized from terminals 48 and 52 through thecontactors 54b and 540. The dryer accordingly commences operation, withthe drum l0 rotating, water being emitted from the jet 34, and theheater 3!] being energized.

As the dryer continues operation, the temperatures in the system riseuntil the temperature sensed by the bulb 44a exceeds the value between30 C. and 50 C. to which the knob 440 is adjusted. At this time, thethermostat contact plate 44d swings from the dotted position of Figure 2to the contact-making solid position, thus estab lishing a circuit inparallel with the thermostat 48 for the energization of the solenoid54a.

As the system temperature continues to rise, the temperature sensed bythermostat 4B exceeds the value of 89 C. to cause that thermostat tosnap from the dotted position to the solid position of Figure 2. In thesolid position of Figure 2, the thermostat breaks its parallel circuitacross the thermostat 44 and establishes an energizing circuit fromterminal 48 to motor 28 and valve 38 independent of contactors 54d.

As the temperature at the outlet of the drum l0 continues to rise, thehigh temperature cutout thermostat operates to prevent temperature inexcess of a safe value. Since this merely interrupts the circuit toheater 30, it does not otherwise alter the operation of the system.

During the operation described above, the clothes are in process ofbeing dried and the rate of moisture removal is determined primarily bythe drying ability of the air introduced into drum l0. The temperaturesensed by the thermostat 44 is above the set value between 30 C. and 50C. and the switch remains closed. However, when the water is removedfrom the clothes, the rate of water removal falls and is reflected inreduced humidity of the air in duct l8. This causes the temperaturesensed by thermostat 44 to fall and eventually drop below the value(between 30 C. and 50 C.) to which this thermostat is set. Thethermostat thereupon drops out to open the circuit to solenoid 54a andcause drop-out of the contactors 54b to Me, inclusive. The heater 30 isthen deenergized.

Upon drop-out of the solenoid operated switch 54, the motor 28 and valve38 remain energized through the thermostat 48. This condition continuesuntil the temperature falls below the low temperature limit of thisthermostat (i. e., 69 C.).

The circuit shown diagrammatically in Figure 3 is a modification of thatof Figure 2 wherein a This switch energizes aeuaao timer I issubstituted for the thermostat ii. The timer 6 is a single pole doublethrow switch exactly like thermostat 46 and connected in the samecircuit relation. However, it operates on a fixed time cycle to maintainthe contact position shown by the dotted lines, Figure 3, for apredetermined period and then to shift to the solid position for asecond predetermined period. The first period is sufficiently long toraise the temperature of thermostat 44 to contact making position andthe second is sufficiently long to exceed the time required for dryingand the after-operation of the motor 28 and valve 38 to cool the unit.

The timer may be any one of many constructions well known in the art andforms no part of the present invention. Its fixed contact arm M61) isconnected to the common terminal of motor 28 and valve 38 by conductor6c, Fig.- ure 3.

Figure 4 shows in chart form the variations in temperature in the dryerwhen it is operated without the controls of the present invention. Inthis figure, curve A represents the temperature of the air entering thedrum which is the temperature sensed by the switch 46, Figure 1. Thecurve B is the temperature of the air entering duct l8, that is, the airentering the spray unit, The curve C is the temperature sensed by thebulb 44a, Figure 1, that is, the temperature of the air as it leaves thespray chamber.

It will be observed from Figure 4 that the temperature of bulb 44a risesfrom about 20 C. to a. peak value of almost 40 C. during the first 25minutes of operation. However, this temperature falls after reachingthis peak value and by the time 55 minutes have elapsed, is well below30 C. Consequently, if the thermostat 44 is set to deenergize the unitat, say, 30 degrees, as indicated by the heavy dashed line, Figure 4,the

unit will be deenergized after about minutes operation.

Experiment has shown that the droop of the curve C after reaching thepeak value is determined by the water content of the clothes being driedand that the temperature value at any time is a measure of the degree ofdryness of the clothes. While applicant cannot state positively thereason for this temperature variation, it is believed to be due to thedecreased moisture content of the air entering duct l8 as the clothesare dried. When the clothes are wet, the water evaporated from themappears as increased humidity of the air in duct IS with the incidentincreased specific heat of the air. The cooling associated withoperation of the spray 34 is accordingly less than it would be if theair contained less moisture and hence had smaller specific heat.

In the appended claims I have used the term aspirator unit to describegenerally the apparatus downstream of the aspirator nozzle 34 andextending to the heater 30. The thermostat 44 may be placed at any pointin this region to sense the degree of dryness of the clothes in tumblingdrum [0, although the location shown in Figure 1 is preferred.

From the above description it will be apparent that I have provided animproved automatic clothes dryer which is energized in a time cycle ofduration determined by the degree of dryness of the clothes and needs nopresetting other than that of the knob c which fixes the degree ofdryness at which the unit cuts oii. Moreover, the unit of the presentinvention is suitable for use with the closed, efficient, automaticdryer of Ralph G. Pugh patent application Serial No. 200,069, entitledClothes Dryer.

It will be observed that the switches 45 and I46 act as conditioningswitches to maintain initial operation of the dryer until the thermostat44 comes up to temperature.

In the appended claims the words jet cooling unit are used to designatea water spray type unit which directly exposes water to the air stream.While the specific jet shown at 3 is also an asplrator, this is notnecessary to the operation of the unit.

While I have shown and described specific forms of the presentinvention, it will of course be understood that many modifications andalternative constructions may be made without departing from the truespirit and scope thereof. I therefore intend by the appended claims tocover all modifications and alternative constructions falling within thetrue spirit and scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a clothes dryer comprising a heater, a drying chamber, means forinjecting a .cooling liquid, and means forming a closed path forcirculating air through said elements in the above order; the mechanismcomprising: a first switch means responsive to the temperature of theair in said drying chamber, a second temperature responsive switch meanslocated downstream of the cooling liquid injecting means with respect tothe air fiow past said liquid injecting means, and means defining anenergizing circuit through said switch means and heater, said firstswitch means being operative through said circuit to energize the heaterindependently of the second switch means until a predeterminedtemperature is reached and being then operative to establish anenergizing circuit through the second switch means and the second switchmeans being operative through said circuit to interrupt the energizingcircuit of the heater when the temperature falls.

2. In a clothes dryer comprising a heater, a drying chamber, means forinjecting a cooling liquid, and means for circulating air through saidelements in the above order; the mechanism comprising: means defining anenergizing circuit for the heater; a switch in said circuit to open orclose the same; and elements responsive to the temperature downstreamthe point of cooling liquid injection operative to open the switch whenthat temperature falls below a preset value.

3. In a clothes dryer comprising a heater, a drying chamber, means forinjecting a cooling liquid, and means for circulating air through saidelements in the above order; the mechanism comprising: means defining anenergizing circuit for the heater; a switch in said circuit to open orclose the same; and elements responsive to the temperature downstreamthe point of cooling liquid injection operative to open the switch whenthat temperature falls below a preset value; and switch means shuntingsaid first switch, and elements operative when set to close said lastswitch means independently of said first switch until the temperature inthe drying chamber exceeds said preset value.

4. In a clothes dryer comprising a heater, a drying chamber having aclothes tumbler with a drive motor, means for injecting a coolingliquid, and means forming a closed path for circulating air through saidelements in the above order: means defining a first energizing circuitfor the motor and heater; means defining a second energizing circuit forthe motor only; a twoposition switch operative in one position to closesaid first circuit; and in the other position to close said secondcircuit; means responsive to the temperature in said drying chamberoperative to move said switch from said one position to said otherposition when the temperature exceeds a predetermined value; a secondswitch connected in electrically parallel relation to the first switchwhen in said one position; and means operative to open said secondswitch when the temperature downstream the point of injecting coolingliquid exceeds a predetermined value.

5. An automatic dryer for clothes and the like comprising incombination: a heater; means defining a clothes drying chamber; meansfor injecting a cooling liquid; means for circulating air through theheater, the chamber, and said means in the above order; means definingan energizing circuit for the heater; a switch in said circuit to openand close the same; and elements responsive to the temperaturedownstream the point of cooling liquid injection operative to open theswitch when that temperature falls below a preset value.

6. An automatic dryer for clothes or the like comprising in combination:a heater; means defining a clothes drying chamber; means for injecting acooling liquid; means for circulating air through the heater, thechamber, and said means in the above order; means defining an energizingcircuit for the heater; a first switch means in said circuit andresponsive to the temperature of the air in said drying chamber; asecond temperature responsive switch means in electrical parallelrelation with said first switch means, said second switch means beinglocated downstream of the cooling liquid injecting means with respect tothe air fiow past said liquid injecting means, said first switch meansbeing operative through said circuit to energize the heaterindependently of the second switch means until a predeterminedtemperature is reached and the second switch means being operativethrough said circuit to interrupt the circuit when the temperature atsaid second switch means falls. 4

7. An automatic dryer for clothes or the like comprising in combination:a heater; means defining a clothes drying chamber; means for injecting acooling liquid; means for circulating air through the heater, thechamber, and said means in the above order; means defining an energizingcircuit for the heater; a setting switch in said circuit; meansresponsive to the temperature in the drying chamber and having arelatively large dead band operative to open the setting switch whensaid temperature rises to a predetermined value; a cut-off switch inelectrical parallel relation to the setting switch; means responsive tothe temperature downstream the cooling liquid injecting means operativeto move the cut-off switch to oil. position when said temperature fallsbelow a predetermined value.

8. An automatic dryer for clothes or the like comprising in combination:a heater; a clothes tumbling drum; means for injecting a cooling liquid;means for circulating air through the heater, the drum, and said meansin the above order; a motor operative to rotate the drum; means definingan energizing circuit for the heater and the motor; a setting switch insaid circuit having an on position and an off position;

' means responsive to the temperature in the drum and having arelatively large dead band operative to move the setting switch to offposition when said temperature rises to a predetermined value; a cut-offswitch in electrical parallel relation to the setting switch when in theon position; means responsive to the temperature downstream the coolingliquid injecting means operative to move the cut-off switch to oilposition when said temperature falls below a predetermined value; andmeans defining an energizing circuit for said motor through the settingswitch in the off position.

9. In combination, a clothes tumbling drum; means defining an air ductexternal to said drum and forming a closed air circulating path inconjunction therewith; means to inject an aspirating spray of coolingliquid into said duct; means located downstream of said last means toremove condensed moisture from the air; a heater located downstream saidlast means; a motor operative to rotate the drum; means defining anenergizing circuit for the heater and the motor; a setting switch insaid circuit and having an off position and an on position; meansresponsive to the temperature in the drum and having a relatively largedead band operative to move the setting switch to off position when saidtemperature rises to a predetermined value; a cut-oiT switch inelectrical parallel relation to the setting switch when in the onposition; means responsive to the temperature downstream the means toinject the aspirating spray operative to move the cut-oif switch to offposition when said temperature falls below a predetermined value; andmeans defining an energizing circuit for said motor through the settingswitch in the oil position.

10. In combination, a clothes drying container;

means defining an air duct external to said container and forming aclosed air circulating path in conjunction therewith; means to inject anaspirating spray of cooling liquid into said duct; means locateddownstream of said last means to remove condensed moisture from the air;a heater located downstream said last means; means defining anenergizing circuit for the heater; a switch in said circuit having anofi position and an on position; and means to move said switch to of!position when the temperature downstream said means to inject theaspirating spray falls below a predetermined value.

ELMER J. PETERSON.

References Cited in the file of this patent UNITED STATES PATENTS Number

